This invention relates in general to voltage controlled oscillators, and more specifically to a method and apparatus for controlling a voltage controlled oscillator.
It is desirable for a voltage controlled oscillator (VCO) used in a portable communication device to exhibit low phase noise in order to satisfy sideband noise requirements. Other applications may also require low phase noise. Prior-art VCOs have operated in open loop, and guaranteeing startup has implied using a high gain in the VCO amplifier. This high gain in the VCO amplifier in turn has produced a large output level. The large output level has resulted in increased phase noise and wasted power, which is unacceptable for portable communications equipment. Additionally, the higher gain needed to guarantee startup requires larger gain devices which increase the parasitic capacitive loading on the tank which in turn decreases frequency and decreases tuning bandwidth. To overcome these problems, some prior-art VCOs have used an automatic gain control (AGC) loop to control startup and the output level. Unfortunately, the AGC loop itself contributes additional phase noise and can cause amplitude instability.